Placement of dopants in MOSFET transistors is increasingly difficult as transistor size is reduced. Misplaced dopants can reduce transistor performance and increase transistor variability, including variability of channel transconductance, capacitance effects, threshold voltage, and leakage. Such variability increases as transistors are reduced in size with each misplaced dopant atom having a greater relative effect on transistor properties consequent to the overall reduction in number of dopant atoms. One common source of misplaced dopants occurs as a result of damage to crystal structure of a transistor during manufacture, which increases defect pathways and creates excess silicon interstitials that allow enhanced dopant movement in undesired regions of the transistor.
A pre-amorphizing implant (PAI) can be used in semiconductor processing as a way to set up for dopant substitutionality. A PAI process generally involves introducing a dopant species using a high energy ion implantation to impart damage and thereby amorphize the implanted region. The damage and amorphization are important so that dopants from a previous or subsequent ion implantation can more easily move into substitutional sites. If PAI is used, then a subsequent anneal is performed to render the substrate crystalline again.
However, with the typical anneal process that is used, which is either a solid phase epitaxy (SPE) or high temperature anneal such as RTA, there remains residual damage from PAI which can create a pathway for leakage current by acting as localized generation/recombination sites.